A hip joint comprises a socket or acetabulum and a femoral head or ball received in the acetabulum. Thus, the hip joint is a ball and socket joint which provides universal motion.
Various diseases, such as osteoarthritis attack the hip joint and, when this occurs, it may be necessary to utilize an appropriate hip joint prosthesis to replace the femoral head and the acetabulum. This may also be necessary in other circumstances such as in the case of certain hip joint fractures.
Deterioration of the acetabulum requires that an acetabular cup be mounted in the acetabulum to provide a socket for slidably receiving the prosthetic femoral head. If the medial wall of the acetabulum is sufficiently weakened, it may be necessary to protect this wall with a protrusio shell. The protrusio shell has one or two flanges for seating on the ilium, the ischium and the pubis. Accordingly, the medial thrust from the femoral head is transferred away from the medial wall through the flanges to the ilium, ischium and pubis.
During surgery, the protrusio shell is cemented into the acetabulum, and immediately thereafter, the acetabular cup is cemented into the acetabulum and within the protrusio shell. Unfortunately, the surgical requirements for separately placing of these two components is quite complex, and it is difficult to achieve the proper positioning of both of these components relative to each other.
The flanges of the protrusio shell, while necessary to achieve the desired force transfer, also tend to interfere with various functions of the hip joint. It is known to provide the flange in two segments, one for the iliac and ischial acetabular rim and the other for the pubic segment of the acetabular rim. One of these spaces between these two segments permits the psoas tendon to glide freely back and forth as it enters the thigh and the other space between the segments eliminates contact between the femoral neck and the flange.
Another problem with hip joint prostheses is dislocation, i.e., removal of the femoral head from the acetabular cup. Some dislocations can be forced back into position by manipulation of the leg under anesthesia and others require open surgery. In either event, a dislocated hip is quite painful.
The segment of the flange between the ilium and the ischial areas creates a likelihood of grinding of the femoral head against the exposed flange in case of dislocation of the hip joint. During relocation of a dislocated hip, the femoral head may hook this flange and cause loosening of the cup.
In an effort to avoid dislocation, it is known to provide an acetabular cup which has straight tangent sections which extend past the hemisphere. Unfortunately, when a hip joint prosthesis of this type dislocates, it usually requires open surgery in order to be placed back into position.
An obvious approach to the dislocation problem would be to extend the acetabular cup far enough past the hemisphere to lock over the femoral head. This would lock the femoral head within the acetabular cup with a snap action which would tend to deter dislocation. Unfortunately, it is necessary that the femoral head be relatively easily removable from the acetabular cup because, if it is not, a dislocation could pull out the acetabular cup from the acetabulum. Accordingly, there is a need for a hip joint prosthesis that will effectively retard dislocation without incurring risk of substantial damage due to forceable removal of the acetabular cup.